An important source of xylene in an oil refinery is catalytic reformate, which is prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. Usually, a C6 to C8 fraction is separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains benzene, toluene and xylenes (BTX), along with ethylbenzene.
Typically, the C6 and C7 hydrocarbons, benzene and toluene, are separated from the C8 aromatic hydrocarbon isomers ethylbenzene and the xylene isomers paraxylene, metaxylene, and orthoxylene. Paraxylene is relatively high value as compared with the other isomers because it is used as the main raw material for polyester fibers. Orthoxylene, useful such as for preparing phthalate esters for plasticizers, is relatively more valuable than metaxylene. Unfortunately, an equilibrium mixture of xylenes contains roughly twice as much metaxylene as para- or orthoxylene.
To recover paraxylene preferentially, typically a C8 aromatic hydrocarbon stream is processed through a paraxylene recovery stage, such as an adsorption process (e.g., a Parex™ or Eluxyl™ absorptive separation unit) or crystallization process, to recover a paraxylene-enriched stream and a paraxylene-depleted stream. The paraxylene-depleted stream can then be catalytically isomerized to equilibrium for recycle in the paraxylene recovery loop. Ethylbenzene needs to be removed from the loop and one way to do so is as explained below.
Generally the catalyst used to promote isomerization of a paraxylene-depleted stream comprises a zeolite supported with a metal component of Group 7-10 of the Periodic Table, e.g., platinum or rhenium. In addition to promoting isomerization between xylene isomers, ethylbenzene can be converted to benzene through a dealkylation reaction and subsequent hydrogenation of the coproduct ethylene, in the presence of such catalysts. One such catalyst is disclosed in U.S. Patent Publication No. 2013/0225891, which teaches a bimetallic catalyst system having two beds. The first bed comprises at least one first metal selected from Groups 7-10, and at least one second metal selected from silver, copper, ruthenium, indium, and tin, dispersed on a silicon-selectivated ZSM-5 molecular sieve. The second bed comprises at least one first metal selected from Groups 7-10, and at least one second metal selected from silver, copper, ruthenium, indium, and tin, dispersed on a non-selectivated ZSM-5 molecular sieve.
However, the quantity of xylenes available from reforming is limited and so recently refineries have also focused on the production of xylene by transalkylation of C9+ aromatic hydrocarbons with benzene and/or toluene over noble metal-containing zeolite catalysts. One process for transalkylation is disclosed in U.S. Patent Publication No. 2013/0259775, in which a C9+ aromatic hydrocarbon feedstock, at least one C6 and/or C7 aromatic hydrocarbon and hydrogen are contacted, under conditions effective to dealkylate aromatic hydrocarbons in the feedstock containing C2+ alkyl groups and to saturate C2+ olefins formed so as to produce a first effluent, with a first catalyst comprising (i) a first molecular sieve having a Constraint Index in the range of about 3 to about 12, and (ii) at least first and second different metals or compounds thereof of Groups 6 to 12 of the Periodic Table of the Elements. At least a portion of the first effluent is contacted with a second catalyst comprising a second molecular sieve having a Constraint Index less than 3 under conditions effective to transalkylate C9+ aromatic hydrocarbons with said at least one C6-C7 aromatic hydrocarbon to form a second effluent comprising xylenes. The first metal of the first catalyst is at least one of platinum, palladium, iridium, and rhenium in an amount between about 0.001 and about 5 wt % of the first catalyst. The second metal is at least one of copper, silver, gold, ruthenium, iron, tungsten, molybdenum, cobalt, nickel, tin, and zinc in an amount between about 0.001 and about 10 wt % of the first catalyst.
Low metal loaded catalysts used for xylenes isomerization, hydrocarbon dealkylation, and hydrocarbon transalkylation, such as those described above, are sensitive to carbon monoxide during both the metal reduction phase of catalyst start-up and normal operation. The impacts of carbon monoxide following start-up of the catalyst is typically reversible (performance returns once carbon monoxide is removed). However, it has been discovered that during conventional start-up procedures in a xylenes isomerization process, the performance of the metal-modified catalyst can be permanently damaged by the presence of carbon monoxide during the initial reduction of the catalyst metal. Specifically, catalysts in xylene isomerization demonstrate higher xylene losses per pass in the reactor if exposed to carbon monoxide during the reduction step. Thus, a method to remove, or at least minimize, carbon monoxide during the start-up phase is desired.
The issue of carbon monoxide poisoning is also encountered with the metal-impregnated catalysts used in reforming. During the regeneration of the reforming catalyst, the metal, typically platinum, may agglomerate during coke removal. A chlorinating agent can be injected with oxygen to re-disperse the agglomerated metal and restore the catalyst chloride level that was reduced during the coke burn. However, carbon monoxide can form in the presence of high carbon dioxide and low oxygen, so the reactor must be purged to reduce the level of oxygen and carbon dioxide therein. Further, the metal must be reduced again due to the formation of metal oxides during the coke burning and metal redispersion steps. A procedure for reducing carbon monoxide prior to agglomeration of the metal of a low metal loaded catalyst is desired.